spinonly

Spinonly refers to a simplified model used to estimate the magnetic moment of transition metal ions and their complexes by considering only the spin contribution of unpaired electrons and neglecting orbital angular momentum. The approach assumes that orbital contributions are quenched by the crystal field in most coordination environments, so the magnetic behavior is governed mainly by electron spins.

In the spin-only model, the effective magnetic moment μeff (in Bohr magnetons) is given by μeff = sqrt(n(n+2)), where n is the number of unpaired electrons. This leads to characteristic values for common ions: for example, high-spin Fe3+ (n = 5) and Mn2+ (n = 5) yield μeff ≈ 5.92 μB, while Ni2+ (n = 2) gives μeff ≈ 2.83 μB. In practice, μeff can be estimated from magnetic susceptibility measurements using μeff ≈ 2.828 sqrt(χM T), where χM is the molar magnetic susceptibility and T is temperature in kelvin.

Spin-only is widely used in coordination chemistry and solid-state magnetism as a first approximation to assign spin states and interpret paramagnetic data, especially for first-row transition metals with strong crystal fields. It provides a quick check against more detailed models and experimental results.

Limitations include the neglect of orbital angular momentum, spin-orbit coupling, and zero-field splitting, which can be significant for heavier transition metals or in environments where quenching is incomplete. Deviations between observed and spin-only values indicate orbital contributions or other interactions beyond the simplifications of the model.